With the recent miniaturization of memory cell transistors MT, the number of electrons trapped in the charge storage layer by a voltage applied to the control gate of the memory cell transistor MT has been decreasing. If the number of trapped electrons varies, this will affect the threshold level of the memory cell transistor MT significantly. That is, a variation in the number of trapped electrons will lead to a decrease in the operation reliability of the memory cell transistors MT as a whole, which has been disclosed in Pat. Appln. KOKAI Publication No. 7-37395.